I just skimmed through that document. Based on your description I was expecting a polemic. But actually about 90% of what they recommend looks like really good ways to improve math instruction.
The big problem to me is that teaching math is hard, and teaching math well, as recommended in that booklet, is really hard. You can write all the pamphlets you want, but in the end the answer will be about the same - get really good teachers. Which probably means paying teachers a lot more than we do now.
The letter appears to be criticizing a different document: The California Mathematics Framework. That framework appears to be around a thousand pages, which I don't have time to read right now, unfortunately.
It is strange that you think that letter would change my opinion though. It doesn't really contain any analysis or arguments. It just says that the changes are bad, and here is a long list of people that agree. After having read the actual document, why would you think reading that letter would change my mind?
Respectfully, the onus is on you to understand what are the perils of such policies that erode the future of our kids, for those that oppose such policies are respected leaders in Mathematics education - perhaps you should question your stance based on what apparently is a marketing pamphelt precisely designed to pursuade people. Based on what you said i.e. "90% of what they recommend looks like really good ways to improve math instruction", I am pointing you to additional information that might convince you otherwise.
Respectfully, I'm having a hard time following your point.
Originally, you linked to a pamphlet and implied that the authors of the pamphlet were "actively undermining the American spirit". I pointed out that the packet appeared to provide good advice for math instruction. You responded "perhaps you should question your stance based on what apparently is a marketing pamphlet precisely designed to persuade people". Why did you link to it then?
What information are you pointing me to that would "convince [me] otherwise"? I read the letter you linked to, and it provides no arguments. As I said previously, it is mostly a list of names of people who are publicly stating their disagreement with a different document - literally appeal to authority fallacy.
I think you would be surprised how much we are aligned on the importance of math education. I'm an engineer from a family of engineers. I clicked your link because I have a son starting kindergarten in a California public school in two and a half weeks. But I was pleasantly surprised to see that the math education being advocated appeared better than the one I received, not worse.
It's important to show what the marketing material is - verbatim - and what kind of BS is being peddled by these people. Apparently, you didn't find it BS so I asked you to read up on it.
I am not following your logic in this entire argument chain. Let's move on.
The big problem to me is that teaching math is hard, and teaching math well, as recommended in that booklet, is really hard. You can write all the pamphlets you want, but in the end the answer will be about the same - get really good teachers. Which probably means paying teachers a lot more than we do now.